In high-speed optical communications, electro-optical modulators are used to modulate a continuous wave (CW) laser output into light pulses that transmit voice, data, and/or video signals over fiber-optic cables. One such modulator is a Mach-Zehnder modulator (MZM). The MZM has a sinusoidal transfer function of modulating voltage in relation to the light output. To operate the MZM in a linear mode providing best optical transmission performance, a DC bias voltage is applied across modulating electrodes of the modulator. Generally, a bias point must be maintained at the point of inflection (i.e., quadrature) of the sinusoidal transfer function. However, the bias point is subject to a drift due to factors such as temperature variations, optical stress, aging of the modulator, and the like. As the bias moves away from the quadrature bias point, the MZM may cause significant intermodulation distortion in the transmitted signal. Therefore, in operation, the bias point should be dynamically controlled.
An established method for controlling the bias voltage of the MZM is based on modulating the bias voltage with an analog (e.g., sinusoidal) pilot tone, detecting a portion of the output optical signal, and detecting a harmonic of the pilot tone using a synchronous detector. The detected pilot tone is then used for generating a feedback signal that adjusts a bias circuit such that MZM operates at a pre-selected bias point. However, this method provides low accuracy near the bias point where the feedback signal is small and excessively sensitive to spurious noise components in the detected pilot tone. These drawbacks result in non-optimal value and instability of the bias voltage of the MZM.
Therefore, there is a need in the art for an improved method and apparatus for controlling a bias voltage of Mach-Zehnder modulators used in high-speed optical communications.